1. Field of the Invention
The present invention relates to a technique for controlling directivity of a speaker array comprised of speaker units, and more particularly, to a delay time calculation apparatus and method for achieving directivity control by adjusting differences between delay times in supplying an input audio signal to speaker units.
2. Description of the Related Art
As a speaker array system, a delay array type speaker array system is known (see, for example, Japanese Laid-open Patent Publication No. 2006-211230). In a delay array type speaker array system, delay times of audio signals supplied to speaker units of a speaker array are adjusted for control of directivity of acoustic waves output from the speaker array. The directivity control is to control the propagating direction and the spread of a combined wavefront of acoustic waves output from the speaker units. Delay times are time differences from when an audio signal output from an acoustic source is received by the speaker array system to when the audio signal is supplied to the speaker units.
In the directivity control disclosed in Japanese Laid-open Patent Publication No. 2006-211230, first delay processing for horizontal control is performed on an input audio signal IN10 to generate n first delayed audio signals corresponding to respective ones of speaker unit columns SP(i, 1), SP(i, 2), . . . SP(i, n) (i=1 to m). Next, second delay processing for vertical control is performed on respective ones of the n first delayed audio signals to obtain n×m second delayed audio signals, which are supplied to the speaker units SP(i, j) (i=1 to m, and j=1 to n).
In an example technique to specify the propagating direction of a combined wavefront, the propagating direction is specified by vertical and horizontal steering angles. Assuming that a direction normal to an array plane of the speaker array is z axis, a vertical direction is y axis, and a horizontal direction perpendicular to the z and y axes is x axis, the propagating direction of the combined wavefront is specified by rotation angles from the z axis to the x axis and from the z axis to the y axis (horizontal and vertical steering angles). In that case, the propagating direction of the combined wavefront can be represented by α and β degrees by which the combined wavefront is steered leftward in the horizontal direction and downward in the vertical direction, thus making it easy to intuitively understand the propagation direction.
In the case of, e.g., a speaker array having four speaker units SP(i, j) (i=1 to 2, j=1 to 2) arranged in two rows and two columns in the horizontal and vertical directions as shown in FIG. 8A, if the horizontal and vertical steering angles α, β are specified as shown in FIGS. 8B and 8C, a combined wavefront propagating in the direction represented by the two steering angles α, β can be generated by controlling delay time differences between audio signals supplied to the speaker units SP (i, j), as described below.
For speaker units disposed adjacently in the horizontal direction (e.g., speaker units SP (1, 1) and SP (1, 2)), audio signals are supplied that have a delay time difference corresponding to a difference between paths of acoustic waves output from these speaker units. For example, with reference to the audio signal for the speaker unit SP (1, 1) (i.e., assuming that the delay time for the speaker unit SP(1, 1) is equal to zero), the delay time for the speaker unit SP(1, 2) is determined to have a value corresponding to a path difference Dx sin α (see FIG. 8B) relative to the speaker unit SP(1, 1). Specifically, the delay time is obtained by dividing the path difference by the sound velocity.
Similarly, for speaker units (e.g., SP(1, 1) and SP(2, 1)) disposed adjacently in the vertical direction, the delay time for the speaker unit SP(2, 1) is determined to have a value corresponding to a path difference Dy sin β (see FIG. 8C) relative to the speaker unit SP(1, 1). Since the speaker unit SP(2, 2) has path differences of Dy sin β and Dx sin α relative to the speaker units SP(1, 2) and SP(1, 1), the delay time for the speaker unit SP(2, 2) is determined to have a value corresponding to the sum of the path differences (Dx sin α+Dy sin β).
With the directivity control in which the propagating direction of a combined wavefront is specified by horizontal and vertical steering angles and delays corresponding to path differences shown in FIGS. 8B and 8C are given, the delay time becomes excessively larger for speaker units which are disposed closer to the corners of the speaker array. As a result, a problem is posed that acoustic waves output from these speaker units do not effectively contribute to the formation of the combined wavefront.
For example, in a case that relations of Dx=Dy=D and α=β=45 degrees are satisfied in the speaker array in FIG. 8A and the sound velocity is represented by C, the delay times for the speaker units SP(i, j) relative to the speaker unit SP(1, 1) are determined as shown in FIG. 8D. It is apparent from FIG. 8D that the delay time for the speaker unit SP(2, 2) becomes excessively large as compared to those for the speaker units SP(1, 2) and SP(2, 1).